In the proposed research, mother-reared and surrogate-reared squirrel monkeys will be either subjected to some form of early separation from the maternal figure, or will not be separated. Both immediate and long-term effects of these procedures will be examined. These experiments are designed to test the hypotheses that: (1) much of the individual variability observed in the long-term effects of maternal separation can be accounted for by individual variability in the strength of infant-mother attachment at the time of separation; and (2) individual variability in attachment strength at the time of separation is influenced by differences in the interactions of infants with their mothers during the first week of life. These experiments will also provide an initial assessment in a primate species of the effects of early maternal separation on later hormonal responses and systematically investigate the behavioral consequences of two forms of early separation in a variety of later tests.